Large pore size molecular sieves are in high demand for reactions or separations involving large molecules and have been sought after for several decades. Due to their low cost, ease of handling, and high resistance to photoinduced corrosion, many uses have been proposed for mesoporous metal oxide materials, such as SiO2, particularly in the fields of catalysis, molecular separations, fuel cells, adsorbents, patterned-device development, optoelectronic devices, and chemical and biological sensors. One such application for these materials is the catalysis and separation of molecules that are too large to fit in the smaller 3-5 xc3x85 pores of crystalline molecular sieves, providing facile separation of biomolecules such as enzymes and/or proteins. Such technology would greatly speed processing of biological specimens, eliminating the need for time consuming ultracentrifugation procedures for separating proteins. Other applications include supported-enzyme biosensors with high selectivity and antigen expression capabilities. Another application, for mesoporous TiO2, is photocatalytic water splitting, which is extremely important for environmentally friendly energy generation. There is also tremendous interest in using mesoporous ZrO2, Si1xe2x88x92xAlxOy, Si1xe2x88x92xTixOy as acidic catalysts. Mesoporous WO3 can be used as the support for ruthenium, which currently holds the world record for photocatalytic conversion of CH4 to CH3OH and H2. Mesoporous materials with semiconducting frameworks, such as SnO2 and WO3, can be also used in the construction of fuel cells.
Mesoporous materials in the form of monoliths and films have a broad variety of applications, particularly as thermally stable low dielectric coatings, non-linear optical media for optical computing and self-switching circuits, and as host matrices for electrically-active species (e.g. conducting and lasing polymers and light emitting diodes). Such materials are of vital interest to the semiconductor and communications industries for coating chips, as well as to develop optical computing technology which will require optically transparent, thermally stable films as waveguides and optical switches.
These applications, however, are significantly hindered by the fact that, until this invention, mesoscopically ordered metal oxides could only be produced with pore sizes in the range (15xcx9c100 xc3x85), and with relatively poor thermal stability. Many applications of mesoporous metal oxides require both mesoscopic ordering and framework crystallinity. However, these applications have been significantly hindered by the fact that, until this invention, mesoscopically ordered metal oxides generally have relative thin and fragile channel walls.
Since mesoporous molecular sieves, such as the M41S family of materials, were discovered in 1992, surfactant-templated synthetic procedures have been extended to include a wide variety of compositions and conditions for exploiting the structure-directing functions of electrostatic and hydrogen-bonding interactions associated with amphiphilic molecules. For example, MCM-41 materials prepared by use of cationic cetyltrimethylammonium surfactants commonly have d(100) spacings of about 40 xc3x85 with uniform pore sizes of 20-30 xc3x85. Cosolvent organic molecules, such as trimethylbenzene (TMB), have been used to expand the pore size of MCM-41 up to 100 xc3x85, but unfortunately the resulting products possess less resolved XRD diffraction patterns. This is particularly the case concerning materials with pore sizes near the high-end of this range (ca. 100 xc3x85) for which a single broad diffraction peak is often observed. Pinnavaia and coworkers, infra, have used nonionic surfactants in neutral aqueous media (S0I0 synthesis at pH=7) to synthesize worm-like disordered mesoporous silica with somewhat larger pore sizes of 20-58 xc3x85 (the nomenclature S0I0 or S+Ixe2x88x92 are shorthand notations for describing mesophase synthesis conditions in which the nominal charges associated with the surfactant species S and inorganic species I are indicated). Extended thermal treatment during synthesis gives expanded pore sizes up to 50 xc3x85; see D. Khushalani, A. Kuperman, G. A. Ozin, Adv. Mater. 7, 842 (1995).
The preparation of films and monolithic silicates using acidic sol-gel processing methods is an active research field, and has been studied for several decades. Many studies have focused on creating a variety of hybrid organic-silicate materials, such as Wojcik and Klein""s polyvinyl acetate toughening of TEOS monoliths (Wojcik, Klein; SPIE, Passive Materials for Optical Elements II, 2018, 160-166 (1993)) or Lebeau et al""s organ ic-inorgan ic optical coatings (B. Lebeau, Brasselet, Zyss, C. Sanchez; Chem Mater., 9, 1012-1020 (1997)). The majority of these studies use the organic phase to provide toughness or optical properties to the homogeneous (non-mesostructured) monolithic composite, and not as a structure-directing agent to produce mesoscopically ordered materials. Attard and coworkers have reported the creation of monoliths with xcx9c40 xc3x85 pore size, which were synthesized with low molecular weight nonionic surfactants, but did not comment on their thermal stability or transparency; see G. S. Attard; J. C. Glyde; C. G. G61tner, C. G. Nature 378, 366 (1995). Dabadie et al. have produced mesoporous films with hexagonal or lamellar structure and pore sizes up to 34 xc3x85 using cationic surfactant species as structure-directing species; see Dabadie, Ayral, Guizard, Cot, Lacan; J. Mater Chem., 6, 1789-1794, (1996). However, large pore size ( greater than 50 xc3x85) monoliths or films have not been reported, and, prior to our invention, the use of block copolymers as structure-directing agents has not been previously explored (after our invention, Templin et al. reported using amphiphilic block copolymers as the structure-directing agents, aluminosilicate mesostructures with large ordering lengths ( greater than 15 nm); see Templin, M., Franck, A., Chesne, A. D., Leist, H., Zhang, Y., Ulrich, R., Schxc3xa4dler, V., Wiesner, U. Science 278, 1795 (Dec. 5, 1997)). For an overview of advanced hybrid organic-silica composites, see Novak""s review article, B. Novak; Adv. Mater., 5, 422-433 (1993).
While the use of low-molecular weight surfactant species have produced mesostructurally ordered inorganic-organic composites, the resulting materials have been in the form of powders, thin films, or opaque monoliths. Extension of prior art surfactant templating procedures to the formation of nonsilica mesoporous oxides has met with only limited success, although these mesoporous metal oxides hold more promise in applications that involve electron transport and transfer or magnetic interactions. The following mesoporous inorganic oxides have been synthesized with small mesopore sizes ( less than 4 nm) over the past few years:
MnO2 (Tian, Z., Tong, W., Wang, J., Duan, N., Krishnan, V. V., Suib, S. L. Science. 
Al2O3 (Bagshaw, S. A., Pinnavaia, T. J. Angew. Chem. Int. Ed. Engl. 35,1102 (1996)),
TiO2 (Antonelli, D. M., Ying, J. Y. Angew. Chem. Int. Ed. Engl. 34, 2014 (1995)),
Nb2O5 (Antonelli, D. M., Ying, J. Y. Chem. Mater. 8, 874 (1996)),
Ta2O5 (Antonelli, D. M., Ying, J. Y. Chem. Mater. 8, 874 (1996)),
ZrO2 (Ciesla, U., Schacht, S., Stucky, G. D., Unger, K. K., Schxc3xcth, F. Angew. Chem. Int. Ed. Engl. 35, 541 (1996)),
HfO2 (Liu, P., Liu, J., Sayari, A. Chem. Commun. 557 (1997)), and reduced Pt (Attard, G. S., Barlett P. N., Coleman N. R. B., Elliott J. M., Owen, J. R., Wang, J. H. Science, 278, 838 (1997)).
However these often have only thermally unstable mesostructures; see Ulagappan, N., Rao, C. N. R. Chem Commun. 1685 (1996), and Braun, P. V., Osenar, P., Stupp, S. I. Nature 380, 325 (1996).
Stucky and co-workers first extended the surfactant templating strategy to the synthesis of non-silica-based mesostructures, mainly metal oxides. Both positively and negatively charged surfactants were used in the presence of water-soluble inorganic species. It was found that the charge density matching between the surfactant and the inorganic species is very important for the formation of the organic-inorganic mesophases. Unfortunately, most of these non-silica mesostructures are not thermally stable. Pinnavaia and co-workers, supra, used nonionic surfactants to synthesize mesoporous alumina in neutral aqueous media and suggested that the wormhole-disordered mesoporous materials are assembled by hydrogen-bonding interaction of inorganic source with the surfactants. Antonelli and Ying, supra, prepared stable mesoporous titanium oxide with phosphorus in a framework using a modified sol-gel method, in which an organometallic precursor was hydrolyzed in the presence of alkylphosphate surfactants. Mesoporous zirconium oxides were prepared using long-chain quaternary ammonium, primary amines, and amphoteric cocamidopropyl betaine as the structure-directing agents; see Kim, A., Bruinsma, P., Chen, Y., Wang, L., Liu, J. Chem. Commun. 161 (1997); Pacheco, G., Zhao, E., Garcia, A., Sklyaro, A., Fripiat, J. J. Chem. Commun. 491 (1997); and Pacheco G., Zhao, E., Garcia, A., Skylyarov, A., Fripiat, J. J. J. Mater. Chem. 8, 219 (1998).
A scaffolding process was also developed by Knowles et al. for the preparation of mesoporous ZrO2 (Knowles J. A., Hudson M. J. J. Chem. Soc., Chem. Commun. 2083 (1995)). Porous HfO2 has been synthesized using cetyltrimethyllammonium bromine as the structure-directing agent; see Liu, P., Liu. J., Sayari, A. Chem. Commun. 557 (1997). Suib et al, supra, prepared mixed-valent semiconducting mesoporous maganese oxide with hexagonal and cubic structures and showed that these materials are catalytically very active. A ligand-assisted templating approach has been successfully used by Ying and co-workers, supra, for the synthesis of Nb2O5 and Ta2O5. Covalent bond interaction between inorganic metal species and surfactant was utilized in this process to assemble the mesostructure. More recently, the surfactant templating strategy has been successfully extended to platinum by Attard, Barlett et al, supra.
For all these mesoporous non-silica oxides (except Pinnavaia""s alumina work, in which copolymers were used to produce mesoporous alumina in neutral aqueous conditions), low-molecular-weight surfactants were used for the assembly of the mesostructures, and the resulting mesoporous materials generally had small mesopore sizes ( less than 4 nm), and thin (1-3 nm) and fragile frameworks. The channel walls of these mesoporous metal oxides were exclusively amorphous. There have been claims, based solely on the X-ray diffraction data, of mesoporous ZrO2 and MnO2 with crystalline frameworks; see Bagshaw and Pinnavaia, supra, and Huang, Y., McCarthy, T. J., Sachtler, W. M. Appl. Catal. A 148, 135 (1996). However, the reported X-ray diffraction patterns cannot exclude the possibility of phase separation between the mesoporous and crystalline materials, and therefore their evidence has been inconclusive. In addition, most of the syntheses were carried out in aqueous solution using metal alkoxides as inorganic precursors. The large proportion of water makes the hydrolysis and condensation of the reactive metal alkyoxides and the subsequent mesostructure assembly extremely difficult to control.
For an overview of the non-silica mesoporous materials prior to this invention, see the Sayari and Liu review article, Sayari, A., Liu, P. Microporous Mater. 12, 149 (1997).
There has also been a need for porous inorganic materials with structure function on different length scales, for use in areas as diverse as large-molecule catalysis, biomolecule separation, the formation of semiconductor nanostructure, the development of medical implants and the morphogenesis of skeletal forms. The use of organic templates to control the structure of inorganic solid has proven very successful for designing porous materials with pore size ranging from angstroms to micrometers. For example, microporous aluminosilicate and aluminophosphate zeolite-type structures have been templated by organic moleculars such as amines. Larger mesoporous (20xcx9c300 xc3x85) materials have been obtained by using long-chain surfactant as structure-directing-agents. Recent reports illustrate that techniques such as surfactant emulsion or latex sphere templating have been used to create TiO2, ZrO2, SiO2 structures with pore sizes ranging from 100 nm to 1 xcexcm. Recently, Nakanishi used a process that combined phase separation, solvent exchange with sol-gel chemistry to prepare macroscopic silica structures with random meso and macro-porous structure; see K. Nakanishi, J. Porous Mater. 4, 67 (1997). Mann and coworkers used bacterial threads as the templates to synthesize ordered macrostructures in silica-surfactant mesophases; see Davis, S. L. Burkett, N. H. Mendelson, S. Mann, Nature, 385, 420 (1997)
Researchers have commented on the assembly of inorganic composites directed by protein or organic surfactants, but little on the effect of inorganic salts on the self-assembly of macroscopic silica or calcium carbonate structures with diatom, coral morphologies; see Davis, S. L. Burkett, N. H. Mendelson, S. Mann, Nature, 385, 420 (1997); A. M. Belcher, X. H. Wu, R. J. Christensen, P. K. Hansma, G. D. Stucky, Nature, 381, 56 (1996); and X. Y. Shen, A. M. Belcher, P. K. Hansma, G. D. Stucky, et al., Bio. Chem., 272, 32472 (1997).
The present invention overcomes the drawbacks of prior efforts to prepare mesoporous materials and mesoscopic structures, and provides heretofore unattainable materials having very desirable and widely useful properties. These materials are prepared by using amphiphilic block copolymer species to act as structure-directing agents for metal oxides in self-assembling systems. Aqueous metal cations partition within the hydrophilic regions of the self-assembled system and associate with the hydrophilic polymer blocks. Subsequent polymerization of the metalate precursor species under strongly acidic conditions (e.g., pH 1), produces a densely cross linked, mesoscopically ordered metal oxide network. Mesoscopic order is imparted by cooperative self-assembly of the inorganic and amphiphilic species interacting across their hydrophilic-hydrophobic interface.
By slowly evaporating the aqueous solvent, the composite mesostructures can be formed into transparent, crack-free films, fibers or monoliths, having two-dimensional hexagonal (p6mm), cubic (Im3m), or lamellar mesostructures, depending on choice of the block copolymers. Heating to remove the organic template yields a mesoporous product that is thermally stable in boiling water. Calcination yields mesoporous structures with high BET surface areas. Unlike traditional sol-gel films and monoliths, the mesoscopically ordered silicates described in this invention can be produced with high degrees of order in the 100-200 xc3x85 length scale range, extremely large surface areas, low dielectric constants, large anisotropy, can incorporate very large host molecules, and yet still retain thermal stability and the transparency of fully densified silicates.
In accordance with a further embodiment of this invention, inorganic oxide membranes are synthesized with three-dimension (3-d) meso-macro structures using simultaneous multiphase assembly. Self-assembly of polymerized inorganic oxide species/amphiphilic block copolymers and the concurrent assembly of highly ordered mesoporous inorganic oxide frameworks are carried out at the interface of a third phase consisting of droplet of strong electrolyte inorganic salts/water solution. The result is a 2-d or 3-d macroporous/mesoporous membranes which, with silica, are coral-like, and can be as large as 4 cmxc3x974 cm with a thickness that can be adjusted between 10 xcexcm to several millimeters. The macropore size (0.5xcx9c100 xcexcm) can be controlled by varying the electrolyte strength of inorganic salts and evaporation rate of the solvents. Higher electrolyte strength of inorganic salts and faster evaporation result in a thicker inorganic oxide a framework and larger macropore size. The mesoscopic structure, either 2-d hexagonal (p6mm, pore size 40xcx9c90 xc3x85) or 3-d cubic array, can be controlled by amphiphilic block copolymer templates. The resulting membranes are thermally stable and have large surface areas up to 1000 m2/g, and pore volume up to 1.1 cm3/g. Most importantly, these meso-macroporous coral-like planes provide excellent access to the mesopore surfaces for catalytic, sorption, catalysis, separation, and sensor arrays, applications.